Abstract: Effect of tempering temperature on microstructure and mechanical properties of a high performance fire-resistant steel was studied by means of optical microscope, transmission electron microscope and mechanical property test. The results show that the tested steel shows good high temperature performance and good match of strength and toughness after tempering, and the fire-resistance and low temperature impact property reach to the optimum when tempered at 650 ℃. Before and after tempering, the microstructure of the tested steel is mainly polygonal F+lath/granular B, and with a small amount of M/A islands, as well as many approximately spherical or elliptical carbides or complex carbonitride. With the increase of tempering temperature, the microstructure slightly coarsens, and some lath B merges into cellular structure. The M/A islands change little in number and morphology, though their effective size slightly decreases. After tempering at 600 ℃, the quantity of precipitated phases increases, and a large number of second phase particles with size below 50 nm precipitate when tempered at 650 ℃ and 680 ℃, which further ensures fire-resistance and low temperature impact property of the tested steel.

Key words: fire-resistant steel, tempering temperature, microstructure, mechanical properties